Organic Light Emitting Diode (OLED) devices can be configured to emit white light and, as such are likely to have utility in providing, inter alia, cheap and efficient lighting, for example for homes, buildings and offices.
In order to provide a white light OLED it is apt to have red, green and blue light-emissive species appropriately provided within the device to ensure that the blend of emitted light is usable for general lighting purposes. The provision of distinct red, green and blue emitters leads performance issues and/or to complexities in device manufacture, for example, insufficient emissions intensities or multiple solvent employing manufacturing steps (which occur in the provision a plurality of distinct emission layers).
In manufacturing electroluminescent devices such as white light emitting devices, multiple electroluminescent layers may be stacked upon one another, whereby each layer comprises an electroluminescent material having an emission spectrum which differs from that of the other layers. For instance, a multi layer device may comprise a red emission layer having a green emission layer deposited thereon, the green emission layer itself having a blue emission layer deposited thereon. However, such multi layer architecture results in devices which have low quantum efficiency and/or experience quenching.
Typically the highest efficiency devices would employ blue, green and red phosphorescent emitters. However, where fluorescent blue is used with phosphorescent green and red, such a multilayer device results in low quantum efficiency and/or experience considerable quenching.
Quenching of phosphorescent green chromophores typically occurs when the energy of the green triplet state is greater than the non-emissive fluorescent blue triplet state. Thus energy is transferred from the phosphorescent green triplet to the fluorescent blue triplet and no longer emits light.
Moreover, solution processed multi layer devices can be time consuming to construct because each layer must be deposited and allowed to dry before the next layer can be applied.
It has been suggested that multiple layers of white light emitting devices based upon fluorescent blue and phosphorescent green emitters can be avoided by employing lateral separation of the fluorescent blue and phosphorescent green emitters. One method of achieving this is by printing discrete areas of different light emitting characteristics, but this requires dedicated printing equipment which is not always available. Another possibility is described in WO 2006/064183 which avoids the need for printing equipment. The method described therein forms a porous layer in a polymer by leaching out a soluble material from a composite layer and then forming an interpenetrating layer of a second polymer. Such a process is cumbersome and a simpler process would be useful.
It is desirable to provide a device which has appropriate performance characteristics and in which manufacture complexity, such as that described in WO 2006/064183, is reduced. Hence, it is desirable to produce, in an OLED device, a single light emitting layer that is quicker and simpler to fabricate, has good quantum efficiency and which experiences acceptable degrees of quenching.